Multiple-blower relative humidity controlled test chamber

ABSTRACT

An accelerated weathering apparatus includes a test chamber ( 30 ), a specimen supporting means ( 34 ), a light source ( 60 ) powered by a power source controlled by a ballast, at least one chamber air temperature sensor ( 26 ), a black panel temperature sensor ( 36 ), and a multiple blower system and control means ( 52 ). A first blower ( 12 ) draws and circulates outside or fresh air and as second blower ( 46 ) optionally draws recirculated air into an air mixing duct. The speeds of the fresh air and recirculated air blowers are independently regulated and controlled by a blower controller based on the chamber air temperature and black panel temperature, respectively. In addition, a humidifier ( 18 ) and humidity controller ( 20 ) regulates humidity within the system as required.

This application claims the benefit of Provisional Application No.60/233,083, filed Sep. 15, 2000.

BACKGROUND OF THE INVENTION

The present invention relates to the art of testing specimens forresistance to deterioration due to sunlight and humidity. It findsparticular application in conjunction with a materials test chamberhaving a controlled multiple-blower system to achieve simultaneous airand black panel temperature control and/or relative humidity control.However, it will be appreciated that the invention has broaderapplications and may be advantageously employed in connection with otheraccelerated weather testing devices and concepts.

In accelerated weather testing, a specimen is supported within a testchamber and exposed to ultraviolet fluorescent lamps, such as xenonlamps. Typically, outside air or fresh air is heated and blown into theinterior of the test chamber in order to regulate the temperature withinthe chamber. In addition, humidity is added to the chamber in the formof evaporated water. In the above-described weathering apparatus, oneexample of the machine's operation includes applying ultraviolet lightrays to one or more specimens of a set temperature for a given period oftime. The lamps are then turned off and the interior of the chamber iskept at the same or a different temperature for a set period of time.Further, humidity may be added to the system in a repeated fashion.Accordingly, specimens are wetted, exposed to ultraviolet rays, anddried in a repeated fashion.

In the weathering system described above, the chamber air temperature(CAT) is regulated using a single blower system, that is a single blowerwhich draws outside or fresh air into the system, along with a damper toregulate air flow. While the single blower system is fairly adequate forcontrolling chamber air temperature, it is inadequate for precisehumidity control as well as simultaneous control of the CAT and blackpanel temperature (BPT). Typically, black panel temperature is measuredusing a temperature sensor placed on the specimen support to measure theactual black panel temperature, that is, the temperature of a darkspecimen disposed within the test chamber. Because a single blower anddamper system is not fully equipped to effectively regulate both chamberair temperature and black panel temperature along with precise humiditycontrol, a need exists for a system and control method for simultaneouscontrol of both CAT and BPT.

The present invention is directed to a multiple blower system andcontrol method for the simultaneous regulation of chamber airtemperature and black panel temperature, which overcomes theabove-referenced problems and others.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an acceleratedweathering apparatus includes a test chamber, a specimen support, and alight source powered by a power source controlled by a ballast. A pairof chamber air temperature sensors disposed before and after the testchamber measure chamber air temperature. A black panel temperaturesensor measures the black panel temperature within the test chamber. Anair heater heats air passing through the system. A dual blower systemdraws and circulates fresh and recirculated air through the system. Thedual blower system includes a fresh air blower and a recirculation airblower. In this apparatus, a method for controlling both air and blackpanel temperature within the test chamber includes selecting a desiredirradiance and selecting both a desired chamber air temperature and adesired black panel temperature. The chamber air temperature and theblack panel temperature are sensed. The sensed chamber air temperatureis compared to the selected chamber air temperature, while the sensedblack panel temperature is compared to the selected black paneltemperature. In response to the comparing steps, the speed of at leastone of the fresh air blower and the recirculation air blower isadjusted.

In accordance with a more limited aspect of the present invention, ifthe sensed black panel temperature is greater than the selected blackpanel temperature, the speed of the fresh air blower is increased.

In accordance with a more limited aspect of the present invention, ifthe sensed chamber air temperature is greater than the selected chamberair temperature, the speed of the recirculation air blower is decreased.

In accordance with a more limited aspect of the present invention, themethod further includes selecting a desired relative humidity andsensing the relative humidity. The sensed and selected relativehumidities are compared and, in response to this comparison, the speedof at least one of the fresh air blower and the recirculation air bloweris adjusted.

In accordance with a more limited aspect of the present invention, ifthe sensed black panel temperature is less than the selected black paneltemperature, the total blower speed is decreased.

In accordance with a more limited aspect of the present invention, ifthe sensed chamber air temperature is less than the selected chamber airtemperature, the fraction of fresh air drawn into the system isdecreased, where the fraction of fresh air is a ratio of the fresh airblower speed to the total blower speed.

In accordance with a more limited aspect of the present invention, ifthe sensed chamber air temperature is less than the selected chamber airtemperature, a blower speed ratio is decreased, where the blower speedratio is a ratio of the fresh air blower speed to the recirculation airblower speed.

In accordance with another aspect of the present invention, anaccelerated weather testing apparatus includes a test chamber, aspecimen supporting means, irradiance sources, a chamber air temperaturesensor, and a black panel temperature sensor. A fresh air blower and arecirculation air blower are controlled by control means, while humidityis controlled by a humidifier and humidity control means. In thisapparatus, a method of accelerated weather testing includes selecting adesired irradiance, chamber air temperature (CAT), black paneltemperature (BPT), and relative humidity. A specimen is irradiated inaccordance with a selected irradiance. Fresh air is drawn into thetesting apparatus with the fresh air blower at an initial fresh airblower speed. Humidity is added to the fresh air. The humidified freshair is heated and circulated through the test chamber. The CAT is sensedas air exits the test chamber and the BPT is sensed. A portion of theair exiting the test chamber is recirculated using the recirculation airblower at an initial recirculation air blower speed such that therecirculated air mixes with fresh air drawn in by the fresh air blower.

In accordance with a more limited aspect of the present invention, themethod further includes comparing the selected CAT to the sensed CAT.The selected BPT is compared to the sensed BPT. In response to thecomparing steps, at least one of the S_(F) and S_(R) are adjusted.

In accordance with another aspect of the present invention, anaccelerated weathering apparatus includes a test chamber and a specimensupporting means for supporting specimens within the test chamber. Alight source, which is disposed within the test chamber, produces lightin the test chamber. A power source powers the light source and iscontrolled by ballast means. Air is circulated through a duct systemwithin the weathering apparatus, where the duct system includes an airmixing duct, in which fresh air and recirculated air mix, a chamberinlet duct disposed between the air mixing duct and a first end of atest chamber, and an exhaust duct disposed between a second end of thetest chamber and an exhaust. At least one test chamber air temperaturesensor is disposed in at least one of the chamber inlet duct and theexhaust duct. A black panel temperature sensor is disposed adjacent thespecimen supporting means for measuring one of black panel temperatureand black standard temperature. A multiple blower system circulates airthrough the test chamber. The multiple blower system includes a freshair blower, which draws room air into the air mixing duct through afresh air inlet, and a recirculation air blower which optionally drawsair from the exhaust duct into the air mixing duct through arecirculation inlet. A blower controller controls the speed of the freshair blower and the recirculation air blower.

In accordance with a more limited aspect of the present invention, theblower controller includes a set-point means for generating and sendinga plurality of set point signals. A comparison processor compares theset point signals to a sensed temperature signal from at least one ofthe black panel temperature sensor and the test chamber air temperaturesensor. A pair of motor controllers control the fresh air and therecirculation air blowers in accordance with signals received from thecomparison processor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an accelerated weatheringapparatus including a controlled multiple blower system in accordancewith the present invention;

FIG. 2 is a diagrammatic illustration of the blower controller inaccordance with the present invention;

FIG. 3 is a flow chart illustrating a method of controlling a multipleblower system in accordance with the present invention;

FIG. 4 is a flow chart illustrating another preferred method ofcontrolling a multiple blower system in accordance with the presentinvention; and

FIG. 5 is a flow chart illustrating another preferred embodiment forcontrolling a multiple blower system in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, an accelerated weathering apparatus 10includes a fresh air blower 12, which draws room air or fresh airthrough a fresh air inlet 14 into an air mixing duct 16. The fresh airtravels through the air mixing duct 16 where a humidifier 18, controlledby a humidity controller 20, adds additional humidity to the air asneeded. Optionally, air heater 22 increases the temperature of the air,if needed, before the air flows into a chamber inlet duct 24. It is tobe understood that the plurality of arrows present in FIG. 1 are toillustrate the pattern of air flow throughout the accelerated weatheringapparatus.

Optionally, the air temperature may be measured by a first airtemperature sensor 26, the operation of which will be described morefully below, before passing into a test chamber 30. The air flows intothe test chamber and over one or more samples 32 disposed on a specimensupporting means 34, such as a sample tray. Preferably, a black paneltemperature 36 sensor is mounted to the specimen supporting means 34.

After passing around the sample tray, the air flows out of the testchamber 30 and into an exhaust duct 40, where a second chamber airtemperature sensor 42 and a chamber humidity sensor 44 measure theexhaust air temperature and either relative humidity or wet bulbtemperature. At this point, a recirculation air blower 46 optionallydraws a portion of the air from the exhaust duct back into an air mixingduct 16 through a recirculated air inlet 48, where it mixes with thefresh air drawn in by the fresh air blower 12, for circulation throughthe system again. Air that is not drawn back into the system through therecirculation air blower 46 flows out of the system through an exhaust50. As is described more fully below, a blower controller 52 controlsthe speeds of the fresh air blower 12 and a recirculation air blower 46in order to control both the chamber air and black panel temperatures.While FIG. 1 illustrates an embodiment containing two air blowers, it isto be appreciated that the present invention is applicable to othermultiple-blower systems.

Prior to performing a test in the weathering apparatus 10, an operatorspecifies or sets the applicable test parameters. Preferably, thedesired irradiance (IRR_(SP)) and at least one of the following: (i) thedesired black panel temperature (BPT_(SP)), and (ii) desired chamber airtemperature (CAT_(SP)) are set. In addition, the desired relativehumidity (RH_(SP)) may be selected by the operator if the test to beperformed requires such. It is to be appreciated that if only one of theCAT and BPT is specified, the other is estimated, either by formula orthrough a lookup table.

Artisans will appreciate that actual chamber air temperature (CAT)cannot be measured directly in the test chamber 30, because of theheating effect of the radiation from the lamps 60. Therefore, chamberair temperature is typically measured at the chamber outlet using thesecond chamber air temperature sensor 42. Alternately, the actualchamber air temperature or dry bulb temperature is measured using anaverage of the temperature readings from the first chamber air sensor26, which is located at the test chamber inlet, and the temperaturereading of the second chamber air temperature sensor located at thechamber outlet. It is to be appreciated that either a weighted or simpleaverage of the temperatures from the first and second chamber airtemperature sensors may be employed.

In one embodiment, the black panel temperature sensor 36 includes anuninsulated black panel sensor, which measures actual black paneltemperature (BPT). Alternately, the black panel temperature sensor 36includes an insulated black panel sensor which measures actual blackstandard temperature (BST). It is to be appreciated that in thebelow-described control methods, BPT and BST may be usedinterchangeably, depending on the requirements of the weathering testbeing performed. In one embodiment, the chamber humidity sensor 44includes a conventional relative humidity sensor. In an alternateembodiment, relative humidity is calculated or looked up based onmeasurements from a wet-bulb temperature sensor, along with temperaturereadings from one or both of the chamber air temperature sensors 26, 42,which provide dry bulb temperatures.

With reference to FIG. 2 and continuing reference to FIG. 1, where likereference numerals refer to like elements, the blower controller 52includes a set point means 70, which receives and stores the desiredtemperature parameters such as BPT_(SP) and CAT_(SP). A comparisonprocessor 74 receives the desired test parameters from the set pointmeans 70 along with CAT readings and BPT readings from the first andsecond CAT sensors 26, 42 and the BPT sensor 36. As is described morefully below, the comparison processor 74 compares the desire testparameters with the measured parameters and sends motor controllersignals to a pair of motor controllers 76, 78, which in turn control thefan speeds of the fresh air blower 12 and the recirculation air blower46.

With reference to FIG. 3, once the weathering apparatus is activated,the irradiance is set 100 and controlled to IRR_(SP) by the lampballasts in a conventional manner. The two-blower embodiment illustratedin FIGS. 1 and 2 is controlled by the blower controller 52. Both theblack panel temperature (BPT) and chamber air temperature (CAT) are set110, 120 for the given test. As air circulates throughout the system,the BPT is measured 130 and compared 150 to the set point to determinewhether or not the BPT is above the set point BPT_(SP). If the BPT isabove the set point, the speed of the fresh air blower is increased 170in order to compensate for the rise in temperature. That is, more freshair is drawn into the air mixing duct through the fresh air inlet by thefresh air blower.

Concurrently, the chamber air temperature (CAT) is measured 140 andcompared 160, 180 to the CAT set point, CAT_(SP). More particularly, ifthe CAT is above the set point, the speed of the recirculation blower isdecreased 190. Further, if the CAT is below the set point 180, the airheater is enabled 195. It is to be appreciated that in this embodimentthe two blowers are controlled by the blower controller as two automaticclosed-loop systems. That is, the speed of the fresh air blower (S_(F))controls and is determined by the BPT, while the speed of therecirculation air blower (S_(R)) controls and is determined by the CAT.Alternately, the blower controller controls the two blowers as twoautomatic closed-loop systems where S_(F) controls and is determined byCAT, while S_(R) controls and is determined by BPT. In this embodiment,as the measured temperatures rise, the respective blowers increase inspeed. In this embodiment, the air heater may be used in conjunctionwith the fresh air blower to provide an additional range for the CAT.

With reference to FIG. 4, in an alternate embodiment, the blowercontroller controls the two blowers as one automatic closed-loop system,with two outputs to control the two blower speeds. In this embodiment,the total blower speed (S_(TOTAL)=S_(F)+S_(R)) controls and isdetermined by the black panel temperature (BPT), while the fraction offresh air (R_(FRESH)=S_(F)/S_(TOTAL)), or a similar weighted ratiocontrols and is determined by the chamber air temperature CAT.

Initially, the desired irradiance is set 200. In addition, the desiredBPT and CAT are set 210, 220. The measured BPT is compared 240 to theset BPT. In addition, the CAT is measured 250 and compared 260 to theCAT set point. In this embodiment, if the BPT is at the set point, andthe CAT is below the set point, the fresh air fraction R_(FRESH) isdecreased 270, while the total blower speed S_(TOTAL) is held constant.In other words, the speed of the fresh air blower is reduced while thespeed of the recirculated air blower is increased. If the BPT is belowthe set point, while the CAT is at or above the set point, the fresh airfraction remains constant while the total blower speed is reduced. Inother words, both the fresh air blower speed and the recirculated airblower speed are decreased. In this embodiment, the air heater may beused to increase the range of temperatures that is achievable.

With reference to FIG. 5, in an alternate embodiment, the total blowerspeed S_(TOTAL) controls and is determined by the BPT, while a blowerspeed ratio (R_(SPEED)=S_(F)/S_(R)), or a similar weighted ratio(controls and is determined by the CAT.)

The radiance is set 300 to the desired value. In addition, the BPT andCAT are both set 310, 320 to their respective desired values. Both theBPT and CAT are measured 330, 350 and compared 340, 360 to therespective set points. In this embodiment, if the BPT is at the setpoint, but the CAT is below the set point, S_(TOTAL) is held constant,while the blower speed ratio R_(SPEED) is decreased 370. In other words,the fresh air blower speed S_(F) is reduced, while the recirculated airblower speed S_(R) is increased. Alternately, if the BPT is below theset point, while the CAT is at or above the set point, the blower speedratio R_(SPEED) remains constant while the total blower speed S_(TOTAL)is decreased, that is, both S_(F) and S_(R) are decreased. In thisembodiment, the air heater may be used to increase the range ofachievable temperatures.

In an alternate embodiment, the blower controller controls the fresh airblower and the recirculation air blower as two open-loop systems. Inthis embodiment, the speed of the fresh air blower and the speed of therecirculation air blower are each independently controlled manually,such as with a potentiometer attached to a motor speed controller. Byadjusting the two blower speeds, the BPT and CAT of the system are eachadjusted, although somewhat interdependently, to fall within specifiedranges. If desired, one or more air heaters are employed in conjunctionwith the fresh air and/or recirculated air blowers to provide a greaterrange of chamber temperatures.

It is to be appreciated that in any of the above-identifiedmultiple-blower temperature control methods, the blower speeds may beheld within fixed maximum and minimum values, and/or within floatingmaximum and minimum values, depending on the operation of each of theblowers. The floating limits are useful because a minimum speed of oneblower is necessary to block the flow from the other blower passing thewrong way through it. For example, if 100% fresh air is required for acertain test, the fresh air blower spins at the speed which provides theneeded airflow. However, if the recirculation air blower is stopped, asignificant amount of fresh air reverse flows through the recirculatedair blower and out the machine exhaust. To prevent this, therecirculation air blower is operated at a slower “blocking” speed,thereby stopping this leakage and providing the full output of the freshair blower to the test chamber.

Further, if desired, once the blower speeds are established, the speedof the fresh air blower may be increased by a nominal amount, 10% forexample, and the recirculation air blower adjusted to yield theequivalent total flow. In this embodiment, the air heater fine tunes theair temperature, yielding more stable temperatures.

Referring again to FIG. 1, the relative humidity within the test chamber30 is controlled using a humidity controller 20, which operatesmanually, semi-automatically, or automatically.

The relative humidity inside the test chamber 30 is controlled using ahumidity controller 20, which operates manually, semi-automatically, orautomatically. The semi-automatic control embodiments require sensingthe relative humidity directly, or calculating it using a sensed wetbulb temperature. A feedback mechanism within the humidity controller 20directs the humidifier 18 to release more humidity as the measuredrelative humidity falls below the specified relative humidity RH_(SP) orless as the RH exceeds RH_(SP). The humidifier 18 takes form in at leastone of a direct water spray, an air-atomized water spray, a mechanicallygenerated water mist, an ultrasonic fog generation, that is, anebulizer, or a water boiler. Further, the humidity controller mayaffect the operation of the two air blowers because relative humidity is“relative” to the air temperature. Therefore, control of the airtemperature is important for controlling the relative humidity even ifthe specified test does not explicitly require temperature control. Forexample, if the RH is below the set point, the recirculation air blowerwill recirculate a higher percentage of air in order to retain andincrease the relative humidity. In contrast, if the relative humidity isabove the RH set point, the fresh air blower draws additional “dry” roomair into the mixing air duct, while the recirculation blowerrecirculates less, and therefor exhausts more, “wet” air from the testchamber.

The invention has been described with reference to the preferredembodiment. Modifications and alterations will occur to others upon areading and understanding of the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. In an accelerated weathering apparatus having a test chamber, aspecimen support, a light source powered by a power source controlled bya ballast, a pair of chamber air temperature sensors disposed before andafter the test chamber, a black panel temperature sensor, an air heater,and a dual blower system for drawing and circulating fresh andrecirculated air, said dual blower system including a fresh air blowerand a recirculation air blower, a method for controlling both air andblack panel temperatures within the test chamber, said methodcomprising: (a) selecting a desired irradiance; (b) selecting a desiredchamber air temperature and a desired black panel temperature; (c)sensing the chamber air temperature; (d) sensing the black paneltemperature within the test chamber; (e) comparing the sensed chamberair temperature the selected chamber air temperature; (f) comparing thesensed black panel temperature to the selected black panel temperature;and (g) in response to comparing steps (e) and (f), adjusting a speed ofat least one of the fresh air blower and the recirculation air blower.2. The method according to claim 1, wherein step (d) includes: sensing afirst air temperature before the test chamber; sensing a second airtemperature after the test chamber; and averaging the first and secondsensed air temperatures.
 3. The method according to claim 2, wherein ifthe sensed black panel temperature is greater than the selected blackpanel temperature, step (g) includes: increasing the speed of the freshair blower.
 4. The method according to claim 3, wherein if the sensedchamber air temperature is greater than the selected chamber airtemperature, step (g) includes: decreasing the speed of therecirculation air blower.
 5. The method according to claim 4, wherein ifthe sensed chamber air temperature is less than the selected chamber airtemperature, the method includes: heating the circulating air using anair heater.
 6. The method according to claim 5, further comprising: (h)selecting a desired relative humidity; (i) sensing the relativehumidity; (j) comparing the sensed relative humidity to the selectedrelative humidity; and (k) in response to comparing step (j), at leastone of: (l) adjusting the speed of at least one of the fresh air blowerand the recirculation air blower; and (m) adding humidity to thecirculating air.
 7. The method according to claim 2, wherein if thesensed black panel temperature is less than the selected black paneltemperature, step (g) includes: decreasing a total blower speed, wherethe total blower speed is a sum of the fresh air blower speed and therecirculation air blower speed.
 8. The method according to claim 7,wherein if the sensed chamber air temperature is less than the selectedchamber air temperature, step (g) includes: decreasing a fraction offresh air drawn into the system, where the fraction of fresh air is aratio of the fresh air blower speed and the total blower speed.
 9. Themethod according to claim 7, wherein if the sensed chamber airtemperature is less than the selected chamber air temperature, step (g)includes: decreasing a blower speed ratio, where the blower speed ratiois a ratio of the fresh air blower speed and the recirculation airblower speed.
 10. A method of accelerated weather testing a specimenwithin a testing apparatus having a test chamber, a specimen supportingmeans, irradiance sources, chamber air temperature sensor, a black paneltemperature sensor, a fresh air blower, a recirculation air blower, acontrol means for controlling the fresh and recirculation air blowers,and a humidifier and humidity control means, the method comprising thesteps of: (a) selecting a desired (i) irradiance, (ii) chamber airtemperature (CAT), (iii) black panel temperature (BPT), and (iv)relative humidity; (b) irradiating the specimen in accordance with theselected irradiance; (c) drawing fresh air into the testing apparatuswith the fresh air blower at an initial fresh air blower speed (S_(F));(d) adding humidity to the fresh air; (e) heating the humidified, freshair; (f) circulating the heated, humidified, fresh air through the testchamber; (g) sensing the CAT of the air as it exits the test chamber;(h) sensing the BPT as the air exits the test chamber; and (i)recirculating a portion of the air exiting the test chamber with therecirculation air blower at an initial recirculation air blower speed(S_(R)) such that it mixes with fresh air drawn in by the fresh airblower.
 11. The method according to claim 10, further comprising: (j)comparing the selected CAT to the sensed CAT; (k) comparing the selectedBPT to the sensed BPT; and (l) adjusting at least one of S_(F) and S_(R)in accordance with steps (j) and (k).
 12. The method according to claim11, wherein if the sensed BPT is greater than the selected BPT, step (l)includes: increasing S_(F).
 13. The method according to claim 12,wherein if the sensed CAT is greater than the selected CAT, step (l)includes: decreasing S_(R).
 14. The method according to claim 11,wherein if the sensed BPT is less than the selected BPT, step (1)includes: decreasing a total blower speed (S_(TOTAL)), where(S_(TOTAL)=S_(F)+S_(R)).
 15. The method according to claim 14, whereinif the sensed CAT is less than the selected CAT, step (l) includes:decreasing a ratio of fresh air drawn into the system (R_(FRESH)), where(R_(FRESH)=S_(F)/S_(TOTAL)).
 16. The method according to claim 14,wherein if the sensed CAT is less than the selected CAT, step (l)includes: decreasing a blower speed ratio (R_(SPEED)), where(R_(SPEED)=S_(F)/S_(R)).
 17. An accelerated weathering apparatuscomprising: a test chamber; a specimen supporting means for supportingspecimens within the test chamber; a light source disposed within thetest chamber for producing light in the test chamber; a power source forpowering the light source; a ballast means connected to the light sourcefor controlling the amount of power the light source receives from thepower source; a duct system through which air circulates within theweathering apparatus, said duct system including: an air mixing duct inwhich fresh air and recirculated air mix; a chamber inlet duct disposedbetween the air mixing duct and a first end of the test chamber; and anexhaust duct disposed between a second end of the test chamber and anexhaust; at least one test chamber air temperature sensor disposed in atleast one of the chamber inlet duct and the exhaust duct; a black paneltemperature sensor disposed adjacent the specimen supporting means formeasuring one of black panel temperature (BPT) and black standardtemperature (BST); and a multiple blower system which circulates airthrough the test chamber, said multiple blower system including: a freshair blower which draws room air into the air mixing duct through a freshair inlet; a recirculation air blower which optionally draws air fromthe exhaust duct into the air mixing duct through a recirculation inlet;and a blower controller which controls the fresh air blower speed(S_(F)) and the recirculation air blower speed (S_(R)).
 18. Theaccelerated weathering apparatus according to claim 17, wherein theblower controller includes: a set-point means for generating and sendinga plurality of set-point signals; a comparison processor which comparesthe set point signals to a sensed temperature signal from at least oneof (i) the black panel temperature sensor; and (ii) the test chamber airtemperature sensor; and a pair of motor controllers which control thefresh air blower and the recirculation air blower, said motorcontrollers receiving control signals from comparison processor.
 19. Theaccelerated weathering apparatus according to claim 18 furthercomprising: a humidifier for adding humidity to circulating air passingthrough the air mixing duct; a humidity sensor for measuring the amountof humidity within the circulated air passing through the exhaust duct;and a humidity controller connected to the humidifier for controllingthe amount of humidity added to the circulating air.